Traditionally, the integrity of remote sensing devices, such as those used in security systems, are checked by determining the presence or absence of the excitation voltage supplied to the sensing device. Typically if the applied voltage corresponds to +V, an integrity check which indicates the presence of +V at the sensing device is interpreted as indicating integrity or lack of tampering with the sensing device. The absence of +V at the sensing device is interpreted as lack of integrity which typically produces an alarm or appropriate control signals. This rather simple and obvious approach to determining the integrity of sensing devices has been circumvented on numerous occasions wherein intruders have neutralized the effectiveness of security sensing devices by connecting a signal source of suitable magnitude and polarity, i.e. +V, across the leads of the sensing device and subsequently disconnecting the sensing device. Thus an integrity check of the sensing device will produce a false +V response which satisfies the requirement of the integrity checking system when, in fact, the sensing device has been disconnected.